In many applications requiring high speed, low noise, and low distortion operational amplifier circuits, dynamic biasing has become a necessity. With a class A amplifier circuit having a pair of differential inputs, for example, dynamic biasing provides more full power bandwidth without increased susceptibility to noise. Under such circumstances, dynamic biasing is able to provide lowered distortion at high operating frequencies without increasing the noise in the input stage. Devices providing dynamic biasing are commercially available in the arts.
A representative example of a dynamic biasing circuit is depicted in FIG. 1 (prior art). The dynamic input range of the prior art circuit of FIG. 1 may be expressed as:VEE+VCESAT+VBE≦VIN DYN≦VCC−VCESAT−VBE   [Equation 1].A graphical representation of the input common-mode range of the circuit of FIG. 1 is also shown in FIG. 2. From the example of the prior art, it can be seen that the input common-mode range of the dynamically biased circuit is bounded by VBE+VCE within the top rail VCC=5V, and bottom rail VEE=0V, respectively. Thus, the dynamically biased amplifier circuit familiar in the arts is limited by its input common-mode range symmetry.
Asymmetric dynamic biasing would be useful in the arts and would provide many advantages. The advantages of asymmetric dynamic biasing include, but are not limited to, particular utility in amplifier applications where it is desirable for the common-mode range to approach or meet the bottom rail. Additional advantages would accrue, such as the extension of low noise, low distortion, and increased slew rates to circuits operating within the lower regions of their input common-mode ranges.